Hi. I am coding a UEL from theory given in a paper. The paper describes two stiffness matrices - tangent and secant stiffness. Tangent is df/du and secant is f/u. As far as I know Abaqus uses tangent stiffness matrix to perform a newton iteration. Have you used secant matrix also in your code anytime ? If yes can you please explain me the logic of incorporating secant matrix in the subroutine. Are there any advantages ?